1. Field of Invention
This invention relates to packages for semiconductor devices in general, and specifically to high performance, low cost pin grid array, ball grid array, and pad grid array packages for semiconductor devices.
2. Discussion of Prior Art
Until quite recently, the standard for high performance semiconductor packages were packages made of alumina ceramic with multiple layers of tungsten conductors within the monolithic ceramic structure. These packages are known as multi layer cofire (MLC) ceramic packages. The chief advantage of this construction is that it provides virtually perfect protection for the semiconductor chip it houses. Most significantly, once sealed, the chip is not exposed to the exterior environment to any significant degree. In other words, MLC packages are highly hermetic. Hermeticity was important to early die because their surfaces were prone to deterioration from the mix of gases and moisture in the normal environment. MLC packages also provided great flexibility of design to accommodate the electrical requirements of high performance die. That design flexibility came from the ability to route the electrical interconnection on several conductor layers. MLC packages also have good thermal performance due to the relatively high thermal conductivity of the alumina ceramic. It is possible to augment the basically good thermal performance through the application of heat spreaders constructed of metal matrix composite of copper and tungsten.
With all of the benefits of MLC packages, there are two issues that are causing a shift to other technology. Those issues are cost and electrical performance. The processes used to manufacture MLC packages are energy intensive and the materials used are expensive. The result is that MLC packages, when manufactured in the highest volumes and with the greatest efficiency, cost around $0.05 per lead for the most common types. This compares to between $0.01 and $0.02 per lead for the most common of semiconductor packages.
The high performance devices that have traditionally used MLC packages, such as microprocessors, have typically been of high enough value themselves to hide the high cost of the packages. If this were the only problem, inertia alone would probably perpetuate the use of MLC packages for many years. The electrical performance issues, on the other hand are driving a switch away from MLC packages. The problem with MLC is in the basic materials set. The dielectric constant of the alumina dielectric is around 10 and the resistivity of the tungsten conductors is in the range of 0.01 .OMEGA./sq. Those properties, combined with the physical scale and relationships of package components yield structures that generate significant noise in both the signal lines and in the power and ground planes at high operating frequencies. That noise is roughly proportional to frequency. The problem of noise, and increasing noise with increasing speed is exacerbated by the fact that as semiconductor technology advances, operating frequencies increase geometrically and operating voltages, and thus noise immunity, decreases.
These problems are driving a search for packaging that provides higher electrical performance while maintaining the thermal performance of MLC, and hopefully reducing price. Currently the most promising candidate for packaging technology to replace MLC is packaging based on laminate, or PC board substrates to provide the multilayer wiring flexibility. The combinations of resinous materials and reinforcing fibers that form the basis for laminate substrates have dielectric constants of less than five and the copper conductors have resistivities of less than 0.001 .OMEGA./sq. The result is a very significant reduction in noise and the ability to design the impedance of signal lines to much more nearly match that of the die. The traditional problem with laminate based packages is low thermal performance of the basic materials set. The copper is a very good thermal conductor but the quantities of copper in a typical package are small relative to the resin and fiber, which have very low thermal conductivity. Current methods of improving the thermal performance of laminate packages involve attaching a copper slug to the package substrate and thermally connecting the chip to the slug either with thermal vias or by attaching the chip directly to the slug.
The practicality of using a metal slug as a thermal management tool in laminate based semiconductor packages has proven to significantly extend the usefulness and applicability of this type of package. While packages with these copper slugs have superior thermal performance, they are not cosmetically acceptable for device that may sell at retail for more than one thousand dollars.
The cosmetic issues have been addressed by some manufacturers by molding a package exterior around the laminate substrate and the metal slug. Packages produced in this manner are functionally equivalent or superior to the more basic laminate based packages and do have a much more finished appearance. The problem is that the molding process is relatively slow and expensive, regardless of the volume or extent of the package that is molded. While the cosmetics are significantly improved over the basic approach, the packages still suffers cosmetically and in terms of perceived value next to MLC ceramic packages. In addition, molded packages require the use of more expensive substrate materials the can stand up to the high temperatures and pressures associated with molding operations.
The objective of the present invention is to provide the maximum possible electrical, thermal, and mechanical performance, plus cosmetics properties that enhance perceived value to the level of high cost competitive technology, and do so for an absolute minimum cost. The package described here accomplishes the performance goals while providing for manufacturing costs that are below those of competing technologies. The present invention achieves those objectives by redefining how and with which components, the package meets its functional requirements. For any package, those basic functional requirements are: 1) protection of the chip itself; 2) translation of the geometries on the chip to those of the system level interconnects; 3) preservation of the electronic potential of the chip; 4) moving heat from the chip to the environment; and 5) to provide a form factor and interface technique compatible with the system level application.
The last requirement is the by the application, and in the case of the present invention, that is area array contacts using pins, solder balls, or conductive pads. Semiconductor packaging, including high performance area array packages, started out by addressing the requirements for protection and geometric translation. The electrical and thermal performance requirements have been relative late comers to the list. With current technology they have been addressed largely by adding features to then current standard package designs. This reactive, evolutionary process has lead to packages with adequate electrical and thermal performance and mechanical properties sufficient for the job, but also to packages employing high cost materials and processes. In addition, the resulting packages, particularly plastic packages, do not have the cosmetic properties consistent with devices that may sell at retail for more than one thousand dollars.
High performance plastic packages utilize metal heat spreaders or heat sinks to deal with the thermal requirements of high performance die. Those heat sinks constitute the mechanically most stable materials in plastic packages. They are not, however, applied in ways to take much advantage of their mechanical properties. In addition to the mechanical and thermal properties of the heat sink, it is also a potential electrical element in the package. Again, in packages based on current technology, only minimal use is made of this resource that must be included in order to address thermal management issues. By converting the heat spreader to be the mechanical foundation of the package and applying relatively unsophisticated forming techniques to the manufacture of the heat spreader, it is converted in the present invention, from a flat piece of metal to an enclosure and mounting system for the die and the electrical interconnect system of the package substrate.
While there are several methods that could be utilized to produce the heat spreader, or case of the present invention, the most cost effective method is forging. By forging the heat spreader, the complex shapes required for the current invention can be produced for very little more than the cost of the current flat heat spreader.
The initial advantages of the forged heat spreader are superior mechanical properties with little or no increase in mass or cost. The mechanical properties of the case 10 shown in FIG. 1, arise from the basic properties of the materials that comprise it, but also from its shape and from the work hardening that results in the forging process.
The ramifications of making the heat spreader the housing for the rest of the package components are far reaching. First of all, with the thermally conductive material covering the entire top and all four sides of the package, the area of effective hear removal surface can be as much as five times that common in competitive technologies. By removing the mechanical requirements of supporting the packages from the substrate, the substrate materials can be selected for their cost and electrical properties.
Conventional flat heat spreaders have traditionally been attached to the package substrate with thermoset epoxies that cover the entire area where the heat spreader and substrate overlap. By replacing the area attachment with well chosen attachment points and replacing epoxy with solder, cost is significantly reduced and the mechanical properties of the packages can be tailored to accommodate the anticipated environmental challenges for the package.
Despite the advantages of current area array, laminate based packages, all of the heretofore known package structures suffer from a number of disadvantages:
(a) The cost of materials and processing is high, even approaching that of the MLC ceramic packages.
(b) The cosmetics of the best of conventional laminate based packages is inferior to that of MLC ceramic.
(c) Expensive materials are required due to processes with high time/temperature integrals.
(d) Thermal performance, in terms of moving heat from the package surface to the surrounding environment is limited.
(e) Electrical performance is not optimized.